• Energy Research

AbstractClimate change-induced glacial melt affects benthic ecosystems along the West Antarctic Peninsula, but current understanding of the effects on benthic primary production and respiration is limited. Here we demonstrate with a series of in situ community metabolism measurements that climate-related glacial melt disturbance shifts benthic communities from net autotrophy to heterotrophy. With little glacial melt disturbance (during cold El Niño spring 2015), clear waters enabled high benthic microalgal production, resulting in net autotrophic benthic communities. In contrast, water column turbidity caused by increased glacial melt run-off (summer 2015 and warm La Niña spring 2016) limited benthic microalgal production and turned the benthic communities net heterotrophic. Ongoing accelerations in glacial melt and run-off may steer shallow Antarctic seafloor ecosystems towards net heterotrophy, altering the metabolic balance of benthic communities and potentially impacting the carbon balance and food webs at the Antarctic seafloor.

During PS85 (in 2014) taxa specific macrofauna density was determined. Sediment from MUC recovered sediment cores and from the benthic chambers was used for macrofauna taxonomical identification and density determination. For these macrofauna analyses only the 0-5 cm horizon from MUC sediment cores and the entire sediment from the benthic chambers was used, sieved over a 500 µm mesh and stored in borax buffered 4 % formaldehyde and stained with Rose Bengal (Heip et al., 1985). Afterwards, macrofauna taxa were identified to the highest taxonomic level and counted.

Measurements of biogeochemical fluxes at the sediment-water interface are essential to investigate organic matter mineralization processes but are rarely performed in shallow coastal areas of the Antarctic. We investigated biogeochemical fluxes across the sediment-water interface in Potter Cove (King George Island/Isla 25 de Mayo) at water depths between 6-9 m. Total fluxes of oxygen and inorganic nutrients were quantified in situ. Diffusive oxygen fluxes were also quantified in situ, while diffusive inorganic nutrient fluxes were calculated from pore water profiles. Biogenic sediment compounds (concentration of pigments, total organic and inorganic carbon and total nitrogen), and benthic prokaryotic, meio-, and macrofauna density and biomass were determined along with abiotic parameters (sediment granulometry and porosity). The measurements were performed at three locations in Potter Cove, which differ in terms of sedimentary influence due to glacial melt. In this study, we aim to assess secondary effects of glacial melting such as ice scouring and particle release on the benthic community and the biogeochemical cycles they mediate. Furthermore, we discuss small-scale spatial variability of biogeochemical fluxes in shallow water depth and the required food supply to cover the carbon demand of Potter Cove's shallow benthic communities. We found enhanced mineralization in soft sediments at one location intermediately affected by glacial melt-related effects, while a reduced mineralization was observed at a location influenced by glacial melting. The benthic macrofauna assemblage constituted the major benthic carbon stock (>87% of total benthic biomass) and was responsible for most benthic organic matter mineralization. However, biomass of the dominant Antarctic bivalve Laternula elliptica, which contributed 39-69% to the total macrofauna biomass, increased with enhanced glacial melt-related influence. This is contrary to the pattern observed for the remaining macrofauna. Our results further indicated that pelagic primary production is able to fully supply Potter Cove's benthic carbon demand. Therefore, Potter Cove seems to be an autotrophic ecosystem in the summer season.

During PS85 in 2014 bacteria density, meiofauna density, macrofauna density and macrofauna biomass was determined. For the bacterial density determination, sediment subsamples were taken with modified syringes (1.17 cm² cross-sectional area) from MUC recovered sediment cores and from benthic chambers. The first centimetre of each sample was stored in a 2 % filtered formalin solution at 4 °C. The acridine orange direct count (AODC) method (Hobbie et al., 1977) was used to stain bacteria in the subsamples and subsequently bacteria were counted with a microscope (Axioskop 50, Zeiss) under UV-light (CQ-HXP-120, LEj, Germany). For the determination of the meiofauna density and identification of meiofauna taxa, sediment subsamples were taken with modified syringes (3.14 cm² cross-sectional area) from MUC recovered sediment cores. The first centimetre of each sample was stored in borax buffered 4 % formaldehyde solution at 4 °C. The samples were sieved over a 1000 µm and 32 µm mesh. Both fractions were centrifuged three times in a colloidal silica solution (Ludox TM-50) with a density of 1.18 g/cm³ and stained with Rose 20 Bengal (Heip et al., 1985). Afterwards, the taxa were identified and counted. Foraminifera are not considered, as the extraction efficiency of Ludox for different groups of foraminifera is insufficient for a quantitative assessment of the group. Therefore, only metazoan meiofauna is recorded. After taking subsamples for bacteria and meiofauna densities, the remaining sediment from MUC recovered sediment cores and from the benthic chambers was used for macrofauna taxonomical identification, and density and biomass determination. For these macrofauna analyses only the 0-5 cm horizon from MUC sediment cores and the entire remaining sediment from the benthic chambers was used, sieved over a 500 µm mesh and stored in borax buffered 4 % formaldehyde and stained with Rose Bengal (Heip et al., 1985). Afterwards, macrofauna taxa were identified to the highest taxonomic level, counted and weighted (blotted wet weight).

Research so far has provided little evidence that benthic biogeochemical cycling is affected by ocean acidification under realistic climate change scenarios. We measured nutrient exchange and sediment community oxygen consumption (SCOC) rates to estimate nitrification in natural coastal permeable and fine sandy sediments under pre-phytoplankton bloom and bloom conditions. Ocean acidification, as mimicked in the laboratory by a realistic pH decrease of 0.3, significantly reduced SCOC on average by 60% and benthic nitrification rates on average by 94% in both sediment types in February (pre-bloom period), but not in April (bloom period). No changes in macrofauna functional community (density, structural and functional diversity) were observed between ambient and acidified conditions, suggesting that changes in benthic biogeochemical cycling were predominantly mediated by changes in the activity of the microbial community during the short-term incubations (14 days), rather than by changes in engineering effects of bioturbating and bio-irrigating macrofauna. As benthic nitrification makes up the gross of ocean nitrification, a slowdown of this nitrogen cycling pathway in both permeable and fine sediments in winter, could therefore have global impacts on coupled nitrification-denitrification and hence eventually on pelagic nutrient availability. In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Lavigne et al, 2014) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation is 2014-12-04.

During PS93.2 (in 2015) bacteria density, meiofauna density, macrofauna density and macrofauna biomass was determined. For the bacterial density determination, sediment subsamples were taken with modified syringes (1.17 cm² cross-sectional area) from MUC recovered sediment cores and from benthic chambers. The first centimetre of each sample was stored in a 2 % filtered formalin solution at 4 °C. The acridine orange direct count (AODC) method (Hobbie et al., 1977) was used to stain bacteria in the subsamples and subsequently bacteria were counted with a microscope (Axioskop 50, Zeiss) under UV-light (CQ-HXP-120, LEj, Germany). For the determination of the meiofauna density and identification of meiofauna taxa, sediment subsamples were taken with modified syringes (3.14 cm² cross-sectional area) from MUC recovered sediment cores. The first centimetre of each sample was stored in borax buffered 4 % formaldehyde solution at 4 °C. The samples were sieved over a 1000 µm and 32 µm mesh. Both fractions were centrifuged three times in a colloidal silica solution (Ludox TM-50) with a density of 1.18 g/cm³ and stained with Rose 20 Bengal (Heip et al., 1985). Afterwards, the taxa were identified and counted. Foraminifera are not considered, as the extraction efficiency of Ludox for different groups of foraminifera is insufficient for a quantitative assessment of the group. Therefore, only metazoan meiofauna is recorded. After taking subsamples for bacteria and meiofauna densities, the remaining sediment from MUC recovered sediment cores and from the benthic chambers was used for macrofauna taxonomical identification, and density and biomass determination. For these macrofauna analyses only the 0-5 cm horizon from MUC sediment cores and the entire remaining sediment from the benthic chambers was used, sieved over a 500 µm mesh and stored in borax buffered 4 % formaldehyde and stained with Rose Bengal (Heip et al., 1985). Afterwards, macrofauna taxa were identified to the highest taxonomic level, counted and weighted (blotted wet weight).